Труды КНЦ вып.5 (ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ вып. 5/2015(31))

UDC 541.135 ELECTROCHEMICAL BEHAVIOR AND KINETICS OF THE INTERVALENCE CHARGE TRANSFER FOR THE SM(III)/SM(II) REDOX COUPLE IN LiF-CaF 2 MELT Yu.V. Stulov1, M. Korenko2, B. Kubikova2, S.A. Kuznetsov1 11. V. Tananaev Institute o f Chemistry and Technology o f Rare Elements and Mineral Raw Materials o f the Kola Science Centre o f the RAS, Apatity, Russia 2Institute of Inorganic Chemistry of the Slovak Academy o f Sciences, Bratislava, Slovakia Absrtact This article is focused on the electrochemical investigation (cyclic voltammetry) of the redox couple Sm(III)/Sm(II) in an eutectic LiF-CaF 2 melt containing SmF3. The first step of reduction for Sm(III) ions, involving one electron exchange in soluble/soluble Sm(III)/Sm(II) system, was found on a tungsten electrode. The study of the Sm(II)/Sm(0) electrode reaction was not feasible, since it's the redox potential is in the same range of the solvent decomposition. The first step was found reversible at temperatures 1075 and 1125 Kup to polarization rate 1 V/s and at temperature 1175 Kthe process was reversible at all applied in this study sweep rates. The diffusion coefficients (D) of Sm(II) and Sm(III) ions were determined by cyclic voltammetry, showing that D decreases when oxidation state increase, while the activation energy of diffusion (Ea) increases. The standard rate constants of charge transfer were calculated for the redox couple Sm(III)/Sm(II) at 1075 and 1125 Kbased on the data of cyclic voltammetry. Keywords: redox couple, samarium, standard rate constant o f charge transfer, diffusion coefficients, cyclic voltammetry. Today there are two scenarios in the spent nuclear fuel management. The first scenario represents the “unclosed” fuel cycle, which is based on the slow long-term cooling of spent fuel in the intermediate repository and then on the idea of the thousands year ultimate storage in underground repository. The second scenario is based on hydrometallurgical reprocessing of spent fuel by PUREX processes (use aqueous solutions and organic extractants) to obtain strengthened new fuel material. Some task is that even the present day PUREX technology is not capable to separate the tri-valent transuranium elements like americium and curium from tri-valent fission products represented by lanthanides (Ln). The existence o f long-lived transuranium elements in the spent fuel results to the environmental problems based on radiotoxicity. However, these long-lived transuranium elements could be in principle incinerated in so-called transmutation reactor systems into short lived or even stabile isotopes. In the present study, electrochemistry of the redox couple Sm(III)/Sm(II) in the LiF-CaF 2 eutectic melt was examined on a tungsten working electrode by cyclic voltammetry. The eutectic mixture LiF-CaF 2 has a wide electrochemical window and suitable melting point (1035 K) and appears to be the more appropriate molten system for separation of actinides and lanthanides. The specific objectives of this study are the determination of Sm(II) diffusion coefficients and standard rate constants of charge transfer for the redox couple Sm(III)/Sm(II). To our knowledge, these data have not previously been reported. Electrochemical tests were performed in the glassy carbon crucible (conical, top ID: 45 mm, bottom ID: 30 mm) laid in a retort made of stainless steel, closed by a detachable flange with built-in holders for the electrodes, thermocouple and inlet and outlet of inert gas. An argon inert atmosphere (99.998%) was used inside the electrochemical cell within the all measurements. An argon gas was previously dehydrated and deoxygenated. The cell was heated using a programmable furnace and the temperature was measured using a Pt/PTRh10 thermocouple. A resistance furnace heats the retort and allowing uniform thermal field in glassy carbon (GC) crucible up to 1373 K. The inner part of the walls of the retort was protected against fluoride vapors by a glassy carbon liner [1]. A tree electrode design has been applied for electrochemical investigations. A glassy carbon crucible served as a counter electrode. Tungsten wire (99.95 % Sigma Aldrich) was used as working electrode (OD: ca 0.8 mm). Platinum wire was utilized as quasi-reference electrode. The surface area of the working electrode was determined after each experiment by measuring the depth of the immersion in the bath mixture (usually 5 mm). The electrodes were interconnected with AUTOLAB (PGSTAT30 potentiostat/galvanostat) controlled by PC with original software (GPES 4.9). The potential scan rate was varied between 5 10 -3 and 2.8 V/s. The electrolytic bath consisted of an eutectic LiF- CaF 2 (77/33 molar ration) salt mixture (LiF - optical grade, LOMO Plc., St-Petersburg, Russia; CaF 2 - reagent grade, NevaReactive, St-Petersburg, Russia). Before use, it was dehydrated by heating under vacuum from room temperature up to the 673 K for several hours. Samarium ions were introduced into the mixture as a samarium fluoride SmF 3 (reagent grade, NevaReactive, St-Petersburg, Russia). The weight of the carried salt (LiF-CaF 2 ) was usually 70 g. The total concentration of samarium was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The cyclic voltammograms obtained at different scan rates (50, 100, 200, 400, 600, 800, ... , 2800 mV/s) on a W working electrode at 1175 K are shown in Fig. The voltammogram indicates one peak in the cathodic region at -1.136 V (vs. Pt quasi-reference electrode) and the anodic peak at 0.811 V (vs. Pt quasi-reference electrode). As can be also seen in Fig. 1, the peak potentials do not change significantly with increasing of scan rate. 279